Const template

[Andrei Alexandrescu (See Website For Email)]

Frits van Bommel wrote:
> Andrei Alexandrescu (See Website For Email) wrote:
>> Frits van Bommel wrote:
>>> Deletion of a const reference will be necessary for const objects at 
>>> the very least (since there are no non-const references to them). 
>>> Presumably, that's what the "const destructor" is for.
>>>
>>> A case could be made for disallowing explicit deletion of const 
>>> references, but that would mean const objects would only be deletable 
>>> by the GC. That, in turn would mean const objects would be unusable 
>>> by people who need (or just prefer) the GC to be disabled...
>>> Perhaps this should be more of a "best practice" instead of a 
>>> language rule?
>>
>> Deletion of pointers to const data has always been a sticky issue in 
>> C++. There are good arguments either way. I think it should be 
>> disallowed, otherwise a caller passing a const class object does not 
>> have a guarantee that the callee didn't mutate the object, which is 
>> the very purpose of the entire scaffolding.
> 
> But as I mentioned, there will only be const references to const 
> instances, so that would mean const objects couldn't be explicitly deleted.
> And there are people who prefer or need to work with GC off. This 
> limitation would deny them the use of const objects (unless they like 
> memory leaks ;) ).

I disagree. If you want to disable the GC, that doesn't mean you can't 
use const. You use a private non-const reference for ownership purposes, 
and you use a const reference for publishing purposes:

Widget owned = new Widget;
const Widget publicized = owned;
... use publicized wherever ...
delete owned;

This is IMHO entirely reasonable.

> For that reason, I think it should maybe merely be very rude to delete a 
> const reference unless you are absolutely sure nobody else has a 
> reference to the object (that they will ever use), not an error.

I think this might be an acceptable solution, but given the choice, I'd 
stay away from it. I think the C++ experience hasn't shown delete const 
to have any desirable virtues.

Imagine a dialog like this:

"You shot a diplomat with immunity!"
"He's dead. A dead body can't have diplomatic immunity."
"But he was alive and immune when you shot him!"
"But at that point my bullet was in the gun."


Andrei